In an electrophotographic image forming apparatus, there is provided a scanning optical apparatus which blinks a laser beam in accordance with an image signal to expose a photoconductor with light. The scanning optical apparatus is configured to deflect a laser beam periodically by a light deflector comprising a rotary polygon mirror and to convert the deflected laser beam into a spot-like image formed on the photoconductor (i.e., target surface to be scanned) such as a photoconductor drum using an imaging optics having f-theta (fθ) characteristics.
With increased requirements for reducing the size and the cost of the scanning optical apparatus, a study has been made to form an imaging optics comprising one lens only. See, for example, Japanese Patent No. 3303558 corresponding to U.S. Pat. No. 5,818,505 (hereinafter referred to as Patent document 1).
In order to highly accurately expose the photoconductor to light using the scanning optical apparatus, it is preferable that curvature of field is successfully compensated over the entire region of the target surface and the spot diameter is uniform. To achieve this, according to the scanning optical apparatus disclosed in Patent document 1, a lateral magnification m in the main scanning direction is specified within a predetermined range.
However, since a conditional formula disclosed in Patent document 1 contains the radius of curvature of the lens surface at the center of the main scanning direction, there is a drawback that the f-theta lens can not be designed using an expression formula in which the radius of curvature of the lens surface in the main scanning plane is undefined.
Further, Patent document 1 discloses a conditional formula for the magnification in the main scanning plane. However, since the position of the back principal point of the f-theta lens within the main scanning plane is not taken into consideration by this conditional formula, the magnification is inaccurate and an actually manufactured f-theta lens may disadvantageously generates jitter greater than estimation. In the conditional formula for the magnification in the main scanning plane, the eccentric amount of the mirror surface of the deflecting mirror, the incident angle of the beam of light onto the mirror surface of the deflecting mirror, and the emission angle of the beam of light from the mirror surface of the deflecting mirror are not taken into consideration. This may also result in that an actually manufactured f-theta lens disadvantageously generates jitter greater than estimation.
In view of the above, it would be desirable to provide a scanning optical apparatus, in which the lens can be designed independently of forms of the formula for defining the lens surface, and by which an occurrence of jitter can be more reliably suppressed.